Currently, implantable medical devices (IMDs) utilize one or more electrically-conductive leads (which traverse blood vessels and heart chambers) in order to connect a canister with electronics and a power source (the can) to electrodes affixed to the heart for the purpose of electrically exciting cardiac tissue (pacing) and measuring myocardial electrical activity (sensing). These leads may experience certain limitations, such as incidences of venous stenosis or thrombosis, device-related endocarditis, lead perforation of the tricuspid valve and concomitant tricuspid stenosis; and lacerations of the right atrium, superior vena cava, and innominate vein or pulmonary embolization of electrode fragments during lead extraction. Further, conventional pacemakers with left ventricle (LV) pacing/sensing capability require multiple leads and a complex header on the pacemaker.
A small sized IMD has been proposed that mitigates the aforementioned complications, termed a leadless pacemaker (LP), that is characterized by the following features: electrodes are affixed directly to the “can” of the device; the entire device is attached to or within the heart; and the LP is capable of pacing and sensing in the chamber of the heart where it is implanted.
The LPs that have been proposed thus far offer limited functional capability. The LP is able to sense in one chamber and deliver pacing pulses in that same chamber, and thus offers single chamber functionality. For example, an LP device that is located in the right atrium would be limited to offering AAI mode functionality. An AAI mode LP can only sense in the right atrium, pace in the right atrium and inhibit pacing function when an intrinsic event is detected in the right atrium within a preset time limit. Similarly, an LP device that is located in the right ventricle would be limited to offering VVI mode functionality. A VVI mode LP can only sense in the right ventricle, pace in the right ventricle and inhibit pacing function when an intrinsic event is detected in the right ventricle within a preset time limit.
It has been proposed to implant sets of multiple LP devices within a single patient, such as one or more LP devices located in the right atrium and one or more LP devices located in the right ventricle. The atrial LP devices and the ventricular LP devices wirelessly communicate with one another to convey pacing and sensing information between each other to coordinate pacing and sensing operations between the various LP devices. However, these sets of multiple LP devices experience various limitations.
For conventional implanted devices such as pacemakers, communication, which is normally RF or inductive communication, is relatively infrequent and is limited to device implantation, patient follow-up, and more recently, patient monitoring. In any of these cases, the time between communication sessions is very large relative to the communication time of any given session. This allows the implanted device to periodically check for the presence of the external instrument and since that external instrument has a significantly better power source (not limited by patient anatomy and related safety issues) it can carry more of the communication burden, offloading to the implant only the minimum function necessary to communicate. For example, the external instrument can transmit stronger signals and can apply more signal processing power to the reception and decoding of the received signals.
In the case of intercommunication between implants, e.g., leadless pacemakers that are configured to fit within a chamber of a patient's heart, however, the power sources are heavily constrained due to patient anatomy and related safety issues. In addition, in order for two or more implantable medical devices to deliver coordinated therapy or perform synchronized data collection, information must be regularly exchanged between the devices, e.g., on a pulse by pulse basis. Any excess power consumption needed for inter-device communication will reduce device battery/power cell longevity, requiring the patient to undergo more frequent operations to have their implants replaced.
Further, communication between an external programmer and an implant, e.g., RF communication, usually permits large transmitter to receiver separations in comparison with the wavelength of communication. Implant to implant communication, on the other hand, may require communication to occur in near-field, i.e., the transmitter to receiver separation may be much smaller than the wavelength of communication, at hundreds of kHz, resulting in a higher noise to signal ratio and requiring higher processing power for decoding the signal.
Additionally, in the case of using conducted communication through the body tissue, the signal is greatly attenuated. Estimates are for a 60 dB to >80 dB attenuation over only a few cm of device separation (with electrode separation of 3 cm). Poor relative orientations can cause even further signal deterioration. A single chamber leadless cardiac pacemaker can receive communication pulses from an external instrument since that instrument can overcome that attenuation with high amplitude pulses. Also the external instrument can tightly control the signal pulse timing to simplify the receiver. If specific messages are lost by distortion or interference, the system can resend the message, with little implication to patient safety.
A dual chamber leadless pacemaker system must overcome the high signal attenuation using lower amplitude (subthreshold) pulses and with limited electrode spacing. There is less control over the electrode orientation so the receiver sensitivity must be much higher to detect and decode low level signals (<1 mV). The signal timing must also be recovered as cardiac events can occur at any time. The communication power budget is more limited and there should be an equal balance between transmitter and receiver complexity as both devices may need to relay information.
There is also a tighter real-time constraint as message information is a part of the active system timing (as compared to reprogramming or diagnostics for an external instrument communication link). The communication link must also be more reliable as the penalty for dropped messages is greater. Due to the tight power constraints, designing a suitable error coding scheme is more challenging since long error codes will consume additional current.
Each of the LP devices must expend significant power to maintain the wireless communications links. The wireless communications links should be maintained in order to constantly convey pacing and sensing information between, for example, atrial LP device(s) and ventricular LP device(s). This pacing and sensing information is necessary to maintain continuous synchronous operation, which in turn draws a large amount of battery/power cell power.
Further, it is difficult to maintain a reliable wireless communications link between LP devices. The LP devices utilize low power transceivers that are located in a constantly changing environment within the associated heart chamber. The transmission characteristics of the environment surrounding the LP device change due in part to the continuous cyclical motion of the heart and change in blood volume. Hence, the potential exists that the communications link is broken or intermittent.